Vapor generator and control therefor

ABSTRACT

The vapor generator provides a uniform and controllable heat input for vaporizing organic working fluid in a vapor engine. A fluid-tight enclosure is partially filled with a heat transfer fluid for transferring heat from a heat source to organic working fluid conducted through the enclosure in a convoluted tube. A simplified pressure responsive temperature control apparatus provides safe and accurate operation.

O United States Patent 1191 1111 3,793,993 Teagan 1 Feb. 26, 1974 [54]VAPOR GENERATOR AND CONTROL 1,880,533 10/1932 Thomas 122/367 THEREFOR[lgLrilakis mson et a Inventor: William g Acton, Mass- 2,363,118 11/1944Chamberlain... 122/33 2,656,821 10/1953 Ray 122/33 [73] Asslgnee' w fxgz mgg 3,055,347 9/1962 Bailey et a1. 122 33 [22] Filed: Sept. 1, 1972Primary Examiner-Kenneth W. Sprague [2]] pp NO: 285,629 Attorney, Agent,or Firm-James L. Neal [57] ABSTRACT [52] US. Cl. 122/33, 122/367 R 51 1Int Cl F22) The vapor generator provides a uniform and controlla- 58Field of Search...l22/32, 33, 367 R, 367 c, ble heat f f gamc l 122/367A a vapor englne. A fluld-tlght enclosure 15 partially filled Wltl'l aheat transfer fluid for transferring heat from a heat source to organicworking fluid conducted [56] References Cited through the enclosure in aconvoluted tube. A simpli- UNITED STATES PATENTS fied pressureresponsive temperature control appara- 3,603,l01 9/1971 Sul1livan12/2/33 X tus provides safe and accurate operation. 3,138,199 6/1964 Be1.... 122 321 X 2,868,178 1/1959 Peters 122/32 11 Clams, 3 DrawmgFlgures 2,791,204 5/1957 Andrus 122/33 PATENTED FEBZB I974 SHEET 1 BF 2VAPOR GENERATOR AND CONTROL THEREFOR BACKGROUND OF THE INVENTION Vaporcycle engines, such as those operating according to the Rankine cycle,have experienced renewed interest. This is in large measure due to theirnonpolluting characteristics, especially when compared with internalcombustion and diesel engines. In developing vapor cycle enginessuitable for modern uses, organic working fluids have emerged as highlydesirable. However, one characteristic of organic fluids, when used as aworking fluid in a vapor engine, which must be carefully dealt with istheir thermal sensitivity. Many good organic working fluids arethermally stable at the normal working temperature of vapor cycleengines, but such fluids are often not stable sufficiently above suchworking temperatures to prevent decomposition from local overheating andhot spots occurring in ordinary vapor generators. Consequently,attention is directed to vapor generators capable of vaporizing organicworking fluid but not overheating the fluid at any point. US. Pat. No.3,477,412 of Sotiris Kitrilakis, assigned to the assignee of the presentinvention, discloses oneapproach to this problem.

It is an object of this invention to provide a highly simplified vaporgenerator suitable for use with organic working fluids.

It is another object of this invention to provide a vapor generator fororganic working fluids which involves an uncomplicated, dependable andsafe control system.

It is a further object of this invention to provide a vapor generatorfor organic working fluids characterized by a high degree of temperatureuniformity in the heat transfer zone to thus avoid overheating of theorganic working fluid.

SUMMARY OF THE INVENTION This invention pertains to a general purposevapor generator particularly adaptable for use with a vapor cycle engineand other systems wherein the working temperature requires very accuratecontrol. An essential feature of the invention resides in a fluid-tightenclosure partially filled with a suitable heat transfer fluid throughwhich there extends a conduit for a second fluid ultimately to be heatedwhich, in the case of a vapor cycle engine, is typically an organicworking fluid. The conduit conducts fluid to be heated through theenclosure in heat exchange relationship with the heat exchange fluid.There is also provided a burner or other suitable heat source means forproducing heat input to the heat transfer fluid. Heat input to the heattransfer fluid produces boiling and transfer fluid vapor fills theenclosure through which the conduit extends. The boiling liquid and theresulting vapor completely immerse the conduit in a fluid medium.

This constitutes a highly efficient heat transfer system. The heattransfer fluid is in a condition of pool boiling which results in anexceedingly high heat transfer coefficient between the heat transferfluid and any portion of the conduit extending into the boiling liquid.Further, portions of the conduit extending above the liquid level of theboiling heat transfer fluid are immersed in the fluid vapor. The fluidtravelling through the conduit is at a lower temperature than thevaporized heat transfer fluid; the heat transfer fluid vapor thuscondenses upon the conduit and gives up its latent heat of vaporization.

The fluid-tight enclosure may be substantially evacuated ofnon-condensables to create an equilibrium or near equilibrium conditionin the enclosure. In this event, the enclosure pressure is solely afunction of the enclosure temperature. Since the enclosure is filledwith fluid vapor and boiling liquid in equilibrium, there is asubstantially uniform pressure and temperature throughout the entireenclosure at any given time. A single pressure measurement thereforewill provide all the needed temperature and pressure information formonitoring and control of the system. For example, a single pressuresignal can be the only signal required to control heat output of theheat source as a function of working fluid temperature. Anotheradvantage resulting from equilibrium conditions internal of theenclosure is that, at start-up, the heat transfer fluid begins to boilimmediately upon being heated by the heat source.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cut-away perspective viewshowing one preferred embodiment of the invention;

FIG. 2 is a cross-sectional view through another embodiment of thisinvention; and

FIG. 3 is a sectional view along lines 3-3 of FIG. 2.

DETAILED DESCRIPTION OF THE DRAWINGS FIG. 1 shows a heat exchanger whichwill be described as a vapor generator for a vapor engine system,although it is suitable for general use. The vapor generator comprises atube housing 102 and an evaporator housing 104, the two housingstogether constituting a single fluid-tight vessel, or enclosure, 105having openings 106 and 108 for purposes which will be subsequentlydescribed. Within the tube housing 102 there is installed afluid-conducting tube 110 having an inlet port 112 and an outlet port114. The tube conducts through the vapor generator tube housing 102 anorganic working fluid for a vapor cycle engine. Con-,

densed working fluid from a vapor engine condenser, not shown, isadmitted in the liquid state to the tube 110 through the inlet port 112.In the vapor generator 100, the working fluid is evaporated and passesfrom the tube 110 through the outlet port 114. It is then admitted to anexpander, not shown, which forms a part of the vapor cycle engine in amanner well understood in the art.

In the evaporator housing 104, there is a heat source means consistingof a plurality of ducts 116 having heat transfer fins 118 therein. Theducts 116 conduct a heating fluid from an appropriate supply through thehousing 104 and constitute the heat source means for the vaporgenerator. The evaporator housing 104 surrounding the ducts 116 containsa heat transfer fluid 120 approximately to the upper level of the ducts116,

as indicated by the line 121. A plate 122 fastened to the end of theevaporator housing 104 is adapted to facilitate connection of the ducts116 to a supply of heating fluid while maintaining a fluid-tight sealbetween the enclosure 105 and the environment. A similar plate, notshown, is located at the opposite end of the housing 104 for permittingdischarge of the heating fluid from the ducts 116.

The inlet ends of the ducts 116 may be connected to the outlet of aburner, not shown, so that products of combustion from the burner willpass through the ducts to heat the transfer fluid 120. The outlet endsof the ducts 116 will then permit appropriate discharge of theseproducts of combusion.

When products of combustion or other suitable hot fluids at atemperature level about the boiling point of the heat fluid 120 and thedesired temperature level of the vapor engine working fluid pass throughthe ducts 116, pool boiling of the heat transfer fluid is produced andthe vapor created thereby fills the tube housing 102. The relativelycool liquid working fluid entering the tube 1 at the inlet port 1 12 isheated through the wall of the tube 110 by condensation of transferfluid vapor upon the outer surface thereof. The working fluid travelsthrough the tube 1 10 with a sufficient residence time therein tovaporize and pass from the vapor generator through the outlet port 114as a working fluid vapor. The residence time may be a function of theworking fluid circulation rate produced in the vapor engine. Thetransfer fluid 120 which condenses on the tube 110 drops back to theliquid pool in housing 104. Thus, it will be appreciated that the tubewall, or the wall of any suitable conduit exposed to the interior of theenclosure 105, defines a condensing surface and serves to confine theultimate fluid to be heated.

A pressure tap 126 to the interior of the tube housing 104 provides areading of the pressure therein by means of a pressure gauge 128. Sincethe pressure in the tube housing 102 is a function of the housingtemperature, the pressure signal may also yield a temperature reading.

In order to maximize the efficiency of a vapor cycle engine, it isnecessary to operate the engine at as high a working fluid temperatureas possible. This implies that the temperature will be close to thosetemperatures at which unacceptable organic working fluid decompositiontakes place. Organic working fluids can be obtained which are thermallystable at the maximum working temperature of the vapor cycle engine.However, most have thermal stability not sufficiently above the maximumworking temperature of the engine to avoid thermal breakdown if localheating within certain zones of the vapor generator exceedssubstantially the maximum anticipated working temperature of the engine.That is to say, most vapor generators are characterized by hot spots inwhich the temperature is substantially above the overall vapor generatortemperature; with organic working fluids, these hot spots have beenfound to produce a wholly unacceptable amount of thermal decomposition.Therefore, for maximum efficiency, the vapor generator must maintain theworking fluid temperature very near that temperature level at whichthermal breakdown will occur while simultaneously avoiding tube walltemperature above that level at any point.

In the vapor generator described above, the temperature in the enclosure105 is substantially uniform, being the temperature of the boiling heattransfer fluid 120 and its vapor. The temperature of the heat transferfluid does not change substantially with momentary fluctuations in thetemperature of the hot gases passing through the ducts 116. Therefore,the temperature of the enclosure 105, and thereby of the condensingsurface formed by the tube 110, can be safely and accurately maintainedat a predetermined level by governing the temperature or volume of hotgases flowing through the ducts 116. This temperature control may be afunction of the temperature and pressure in the enclosure 105. It may beaccomplished by connecting the pressure tap 126 to a conventionalpressure responsive burner control system substantially in the mannerfully explained in connection with the apparatus of FIG. 2. As afail-safe measure, a frangible plug 130 in the wall of the enclosure isprovided as a safety release in the event the pressure and temperaturein the tube housing approaches dangerously high levels.

Temperature uniformity of the system and controllability of the systemare enhanced if the fluid-tight enclosure' 105 is at least partiallyevacuated of noncondensables until the pressure therein is notsubstantially above the vapor pressure of theheat transfer fluid 120.The enclosure 105 with the heat transfer fluid therein may be evacuatedby a vacuum pump and then sealed. Evacuation also removes potentiallycorrosive water vapor and permits construction from ordinary andinexpensive materials such as steel or cast iron. Further, underevacuated conditions, temperature and pressure throughout the enclosurewill always be essentially uniform and the pressure in the enclosurewill be a function only of the temperature in the enclosure. The liquidand vapor temperature is easily measured at any single point. It may bemeasured directly by a single thermocouple or thermal switch mountedanywhere in the evacuated enclosures. Alternately, the vapor pressure inthe enclosure, which provides a direct measure of temperature, may bemeasured and used toprovide a system control, in the manner referred toabove and described in greater detail in connection with the apparatusof FIG. 2.

Since the temperature is uniform througout the evacuated enclosure,there is no possibility of localized hot spots damaging the vapor engineworking fluid in the tube 110. The temperature of the working fluid cannot exceed that of the heat transfer fluid 120. Therefore, absolutecontrol of the maximum temperature of the working fluid and positiveprevention of its degradation is achieved. Control of the transfer fluidtemperature and residence time of the working fluid in the enclosure 105permits a preselected working fluid temperature to be attained. Further,when the enclosure is evacuated, the evaporator housing 104 may beconfigured so that it holds only a small amount of heat transfer fluid120. In this event, the heat transfer fluid heats very rapidly, thusreducing to a minimum the start-up time for the vapor generator andtherefore of the vapor engine. For example, one experimental vaporgenerator will provide start-up time of approximately 60 seconds.

The vapor generator 10 of FIG. 2 comprises basic as semblies forming aburner 12, combustion chamber 14, an enclosure 16 in which heat transfertakes place, and an exhaust passage 18.

The burner 12 is mounted above the combustion chamber 14 and includes anozzle 20 for directing fuel into the combustion chamber and a blower 22for providing a supply of air. The fuel and air constitute thecombustible mixture which burns in the combustion chamber. Swirlerblades 24 surround the nozzle, 20 to provide turbulence within thecombustion chamber.

The combustion chamber 14, enclosure 16 and exhaust passage 18 are allformed integrally as described below. An outer cylindrical wall 26defines one wall of the enclosure 16 and the exhaust passage 18 while aninner cylindrical wall 28 forms a second wall of the enclosure andexhaust passage. The upper part of the enclosure 16 is formed by anannular wall 30 and the lower portion thereof by a similar annular wall32. The annular wall 32 divides the enclosure 16 from the exhaustpassage 18, the exhaust passage being below and coextensive with theenclosure. Inside the inner cylindrical wall 28 is the combustionchamber 14 in which fuel and air are directed from the burner 12. Theenclosure 16 is partially filled with a heat transfer fluid 34 and, aswith the apparatus of FIG. 1, preferably evacuated of non-condensables,such as air, until the pressure in the enclosure is at leastsubstantially down to or below the vapor pressure of the heat transferfluid.

Between the combustion chamber 14 and the exhaust passage 18, there isprovided a port 36 for permitting products of combustion to pass fromthe combustion chamber 14 to the exhaust passage 18. Referring to FIG.3, it can be seen that the combustion products divide and pass alongboth sides of the annular exhaust passage and are discharged through aport 38 in the outer wall 26. There are extending down into the exhaustpassage 18 from the lower wall 32a plurality of heat transfer fins 40.The heat transfer fins are in a generally plate-like configuration andshaped to guide the flow path of the combustion products through theexhause passage 18.

FIG. 3 illustrates a fin configuration which directs the combustionproducts flow in a generally semi-circular fashion around both sides ofthe annular combustion chamber, conforming essentially to the flow pathwhich the combustion products would naturally pursue. It should beunderstood, however, that other flow paths may be produced by the fins40.

Heat transfer fins 42 extend upwardly from the bottom annular end wall32 into the heat transfer fluid 34. The heat transfer fins 40 and 42provide, respectively, enhanced heat transfer between the products ofcombustion and the bottom end wall 32 and between the bottom end walland the heat transfer fluid 34.

A convoluted tube 44 is wound in spiral fashion through the enclosure 16for conducting vapor engine working fluid therethrough. At ambienttemperature, the spiral tube is partly immersed in condensed heattransfer fluid and partly above the level of the liquid heat transferfluid. Working fluid may enter the tube 44 in the liquid form at aninlet 46 and pass from the vapor generator to vapor state at an outlet48. The tube 44 may have associated therewith external heat transferfins 50 and internal heat transfer fins 52 for enhancing its heattransfer characteristics.

The walls of the vapor generator are insulated to retain heat andincrease its efficiency of operation. Preferably,-all wall surfaces ofthe enclosure 16 not exposed to combustion products from the combustionchamber 14 are insulated, as well as certain wall surfaces of theexhaust passage 18. Cylindrical insulating sheets 54 cover the outercylindrical wall 26 and insulating sheet 56 extends across the bottomportion of the combustion chamber 14 and the exhaust passage 18. Theinsulating sheet 56 may be supported by an end wall 58. The upperportion of the inner cylindrical wall 28, above the liquid level of theheat transfer fluid 34,

- is also insulated by a cylindrical insulating member 60.

It is desirable to insulate the portion of the wall 28 of the evacuatedchamber 16 exposed to the combustion chamber 14 which is not immediatelyadjacent heat transfer fluid in the liquid state to prevent overheatingof that portion of the wall. By way of further explanation, the portionof the inner cylindrical wall 28 immediately adjacent heat transferfluid 34 in the liquid state is in good heat transfer relationship tothe heat transfer fluid and heat energy is easily transferred throughthe wall 28 to the fluid 34. However, the portion of the wall 28 abovethe liquid level of the heat transfer fluid 34 is contacted interior ofthe enclosure 16 only by the zone filled with a certain amount of heattransfer fluid vapor, this vapor not constituting a good medium fortransferring heat from the wall 28.

The burner 12 is controlled by a burner control means 13 whichcommunicates through a pressure line 15 to the evacuated enclosure 16.The burner control means 13 is responsive to pressure in the enclosurethrough the line 15 so that the firing rate of the burner 12 is directlya function of the pressure and temperature levels in the enclosure 16.The burner control means may be any suitable device capable of varyingthe output of the blower 22 and the nozzle 20 in accordance with thepressure signal received through line 15. There is provided a pressuregauge 17 for giving visual or audible signals corresponding to thepressure level in the evacuated enclosure 16.

Operation of the vapor generator 10 will now be described. As in theapparatus of FIG. 1, working fluid is exhausted as a liquid from a vaporengine condenser (not shown) and subsequently vaporized and fed as avapor to a vapor engine expander (not shown). The working fluid istransported through the vapor generator 10 by the tube 44, liquidworking fluid from the condenser being received by the inlet port 46 andleaving as vapor through the outlet port 48.

As the working fluid travels through the tube 44 it is vaporized byproducts of combustion from the combustion chamber 14. The blower 22directs air from the atmosphere or another suitable supply throughswirler blades 24 into the combustion chamber. Simultaneously, thenozzle 20 injects a supply of fuel. The fuel and air constitute acombustible mixture which, when ignited in a suitable manner, burns toproduce hot gases which pass through the port 36 and travel along theexhaust passage 18 to the exhaust port 38. As the combustion productspass through the exhaust passage 18, they encounter heat transfer fins40 and the bottom annular end wall 32 of the enclosure 16. Combustionproducts also engage insulating sheet 56 and the outer cylindrical wall26 which is insulated by insulating cylinder 64. The insulating members54 and 56 minimize heat loss and a maximum amount of heat is transferredthrough the annular end wall 32. The heat is transferred both directlyfrom the end wall 32 to the heat transfer fluid 34 and also from heattransfer fins 32 to the heat transfer fluid. The heat transfer fluid 34is heated to boiling and the enclosure 16 is filled with the vapor ofthe fluid 34.

The working fluid and the tube 52 are at a lower temperature than thetemperature of the heat transfer fluid. Accordingly, heat is transferredto the working fluid through the walls of the tube 44. Depending uponthe location of the tube 52 within the enclosure 16 and the level of thesurface of the fluid 34, heat is transferred by either or both of twooperations. Heat may be transferred directly through the walls of thetube 52 by the boiling working fluid when the tube or a portion thereofis immersed in the pool of boiling working fluid.

When the tube or portion of it is located above the liquid level of thepool of heat transfer fluid, it is heated by condensation thereon of therelatively hot heat transfer fluid vapor to which it is exposed. Whenthe heat transfer fluid condenses on the surface of the tube 52, itgives up its latent heat by vaporization, which is then transferredthrough the tube walls to the working fluid. Both methods of heattransfer are highly efficient. However, efficiency may be increased byproviding more area over which heat transfer may occur. To achieve theincreased area, the embodiment of FIG. 2 shows heat transfer fins 50extending outwardly from the tube 44 into the enclosure 16 and heattransfer fir 52 extending from the inner surface of the tube 44. Heattransfer to the working fluid vaporizes the work ing fluid during itstravel through the tube 52 so that a hot vapor is discharged through theoutlet 48. Residence time of the working fluid in the vapor generator 10is preferably sufficient to allow the working fluid vapor tosubstantially reach the heat transfer fluid temperature.

The burner control means 13 can be set by a selector means 19 tomaintain a predetermined working fluid vapor temperature, for example700 F. Thereafter, if the working fluid begins to rise above thistemperature, there will be a corresponding rise in pressure interior ofthe enclosure 16. The pressure in the evacuated enclosure 16 is directlyproportional to the temperature therein. In response to the pressureincrease, and the corresponding temperature increase, the burner controlmeans 13 will reduce the heat output of the burner 12. For example, thepressure increase may trip a pressure switch which will deenergize asolenoid to reduce or terminate fuel flow while simultaneously adding aresistance in the fan circuit to slow down or stop the fan and therebyreduce or terminate air flow. On the other hand, if working fluid vaportemperature begins to drop, a corresponding drop in enclosure 16pressure will produce a reversal of the aforesaid events and result inan increased heat output of the burner 12. By use of the pressuresignal, response delay is eliminated. That is, a signal which is afunction of temperature, is given immediately with temperature change.As stated earlier, devices which measure temperature directly may beused, but they often do not provide immediate response to temperaturechange. The pressure control also constitutes an effective safety devicefor the vapor generator in that it will always reduce or cut off theheat input as the pressure internal ofthe enclosure increases above apredetermined level, thus avoiding a dangerous pressure build-up. As afail-safe device, a

heat fusible plug 55, or the like, is provided to avoid excessivepressure build-up in the enclosure 16. A control system similar to thatshown in FIG. 2 may be associated with the apparatus of FIG. 1 tocontrol either the output of heat source or the flow rate of the hotgases through ducts 116.

Any heat transfer fluids which have thermodynamic properties suitable tothe specific application to which the particular vapor generator is putmay be used in vapor generators of this invention. A primary requirementis that the heat transfer fluid be thermally stable at the maximumanticipated operating temperature of the vapor generator. Otherproperties which are desirable are the possession of a high condensingheat transfer eoefficient and a vapor pressure low enough thatcontainment of it is not excessively expensive or potentially dangerousat the highest anticipated working temperature. On the other hand, it isnot required that the heat transfer fluid be thermally stable at atemperature level substantially above the maximum anticipated operatingtemperature of the vapor generator because of the extreme ease withwhich the temperature and pressure conditions in the enclosure 16 aremonitored and controlled.

One heat transfer fluid which has proved highly successful in operationof the vapor generator for a vapor cycle engine is Dowtherm A(biphenyland biphenyl I oxide), manufactured by Dow Chemical Company,lo-

cated atMidland, Michigan 48640. Dowtherm A has a good condensing heattransfer coefficient. Fo r exarn ple, under a typical condition workingfluid enters the vapor generator at 380 F and exists at 550 F while theDowtherm A is maintained at a temperature of 600 F. The heat transfercoefficient is then 280 Btu/hr-ft F at entrance and 200 Btu/hr-ft F atexit when a 7% inch OD tube is used. Another advantage of Dowtherm A isthat it contracts when it freezes and thereby does not damage the vaporgenerating equipment. It is also thermally stable above the maximumworking temperature for most vapor cycle engines, for example, above 700F. At 700 F, Dowtherm A has a relatively low vapor pressure ofapproximately 107 psia. At F, taken as typical ambient temperature, itsvapor pressure is less than 0.01 psia; at 550 F, a typical operatingtemperature, the vapor pressure is 27.59 psia. At 496 F, the vaporpressure is 14.7 psia; at 600 F it is 45.55 psia. Accordingly, it willbe seen that a vapor generator utilizing Dowtherm A can operate overmost of its range with a subatmospheric pressure, and in any event, withvery low pressure internal of the enclosure 16. Further, it isreportedly thermally stable at temperature of around 800 F so there isno problem with thermal decomposition in the vapor generators describedabove. However, whether used as a working fluid or heat transfer fluid,its thermal stability is not high enough to tolerate hot spots withoutalso encountering local thermal decomposition at these hot spots.

Other suitable organic heat transfer fluids are Dowtherm E(ortho-dichlorohenzene), also manufactured by Dow Chemical Company;flurocarbons FC-75 and FC-4 manufactured by Minnesota Mining andManufacturing Co. of St. Paul, Minnesota; blem sh- 5mm naphthalene) fromSoken Chemical Engineering Co., Ltd. of Japan; KSK oil from KurekaChemical Co., Ltd. of Tokyo, Japan and Gardena, California; Anisole(phenyl methyl ether); and para-methyl isopropyl benzene. Furtherinformation concerning the above fluids may be found in Handbook of HeatTransfer Media, Paul L. Geiringer; Reinhold Publishing Corp., N.Y.(1962). Toluene, hexafluorobenzene and trifluoroethanol may also be usedas heat transfer fluids.

In addition to the organics, certain inorganics such as stenictetrachloride, tetrametlyltin and tetramethylgermane may be used.

The vapor generator is sufficiently versatile that a wide range of otherfluids may be used. For example, it will accommodate use of water as aheat transfer fluid. However, there are disadvantages. For example,water is corrosive relative to the organics and has a vapor pressure ofapproximately 1,100 psia at 550 F. A working fluid exit temperature of700 F is associated with a water vapor pressure of over 3,200 psi.Containment of water vapor at this temperature requires a relativelyheavy construction and is a potential safety hazard. If relatively highoperating temperatures are always required, certain metals may be usedas heat transfer fluids, for example, mercury and tin. It should bepointed out that a fluid having a very low pressure might not bedesirable. For example, if the vapor pressure at 550 F should be in thevicinity of l or 2 psi, the heat transfer properties of the fluid mightbe reduced below levels generally considered acceptable.

The present invention has been described in reference to preferredembodiments. It should be understood that modifications may be made bythose skilled in the art without departing from the scope of theinvention.

I claim:

1. A vapor generator for a vapor engine using organic working fluid,said vapor generator comprising:

a. means forming a sealed, fluid-tight enclosure of annularconfiguration forming a combustion chamber in the center of the annulus;

b. heat transfer fluid non-decomposable at the anticipated maximumworking temperature of said vapor generator partially filling saidenclosure in the liquid state, said enclosure being at least partiallyevacuated of non-condensables to establish a pressure level therein notsubstantially above the vapor pressure of said heat transfer fluid;

c. conduit means for conducting organic working fluid into heat exchangerelationship with the interior of said enclosure; and

d. heat source means for directing a combustible mixture into saidannulus and pool boiling said heat transfer fluid at a temperature levelequivalent to or above the boiling point of said organic working fluidwhereby heat energy is transferred through the wall of said conduitmeans to said organic working fluid.

2. A vapor generator according to claim 1 wherein said annular enclosurecomprises concentric side walls and top and bottom annular'end walls,further comprismg:

a. an annular exhaust passage substantially coextensive with saidannular enclosure and extending downwardly from said bottom end wall;

b. means forming at least one entrance opening between said combustionchamber and said exhaust passage to admit combustion products to saidexhaust passage; and

0. means in said exhaust passage forming at least one exit openingremote from said entrance opening for discharging exhaust gases, wherebyexhaust gases travel from said combustion chamber, along a path adjacentthe bottom end wall of said enclosure, before being discharged.

3. A vapor generatr according to claim 2 wherein said exit opening issituated 180" from said entrance opening.

4. A vapor generator according to claim 2 further comprising:

a. heat transfer fins extending upward from said bottom end wall intosaid heat transfer fluid; and

b. heat transfer fins extending downward from said bottom end wall intosaid exhaust passage.

5. A vapor generator according to claim 4 wherein said downwardlyextending heat transfer fins comprise elongated plate-like members fordirecting the flow path in said exhaust pgssage.

6. A vapor generator according to claim 2 further comprising thermalinsulation along the common wall between said enclosure and said annularspace restricted substantially to the portion of said common wall abovethe level of liquid in said enclosure.

7. A vapor generator according to claim 1 further comprising meansresponsive to the pressure in said enclosure for controlling the heatinput to said heat transfer fluid, to thereby maintain a predeterminedoperating temperature level in said vapor generator.

8. A vapor generator according to claim 1 further comprising:

a. pressure actuated means for controlling the heat input to said heattransfer fluid;

b. means for applying a signal proportional to the pressure internal ofsaid enclosure to said pressure actuated means, said signal providingthe total information required for enabling said pressure actuated meansto variably control said heat input and maintain working fluid vapordischarged from said vapor generator at a predetermined temperaturelevel.

9. A vapor generator for closed cycle vapor engine utilizing a thermallysensitive organic working fluid, said vapor generator comprising:

a. means forming a substantially rigid, sealed, fluidtight enclosure ofsubstantially constant volume, substantially devoid of fluids which arenoncondensable over the working temperature range of said vaporgenerator, partially filled throughout said range with a heat transferfluid in the liquid state, said heat transfer fluid beingnondecomposable throughout said range;

b. heat source means for vaporizing a portion of said heat transferfluid;

c. conduit means extending through said enclosure with at least aportion thereof out of contact with said heat transfer fluid in theliquid state and forming a condensing surface upon which vaporized heattransfer fluid condenses to transfer thereto its latent heat ofvaporization, said conduit means having an inlet for admitting workingfluid into heat exchange relationship with the interior of saidenclosure and an outlet for discharging working fluid from said heatexchange relationship; and

d. pressure responsive control means, responsive only to the vaporpressure within said enclosure, coupled to said heat source means forcontrolling the output thereof to maintain the heat transfer fluidvapor, and thereby the entire condensing surface, at a temperature levelnot lower than the boiling point of said working fluid and below thetemperature at which an unacceptable amount of thermal degradation ofsaid working fluid occurs, whereby said vapor generator is enabled toheat working fluid at said outlet to vaporization without producingthermal decomposition thereof.

10. A heat transfer device according to claim 9 wherein said heattransfer fluid comprises biphenyl and biphenyl oxide.

11. A heat transfer device according to claim 9 wherein said pressureresponsive means comprises a variable control for selecting theoperating temperature of said condensing surface.

1. A vapor generator for a vapor engine using organic working fluid,said vapor generator comprising: a. means forming a sealed, fluid-tightenclosure of annular configuration forming a combustion chamber in thecenter of the annulus; b. heat transfer fluid non-decomposable at theanticipated maximum working temperature of said vapor generatorpartially filling said enclosure in the liquid state, said enclosurebeing at least partially evacuated of non-condensables to establish apressure level therein not substantially above the vapor pressure ofsaid heat transfer fluid; c. conduit means for conducting organicworking fluid into heat exchange relationship with the interior of saidenclosure; and d. heat source means for directing a combustible mixtureinto said annulus and pool boiling said heat transfer fluid at atemperature level equivalent to or above the boiling point of saidorganic working fluid whereby heat energy is transferred through thewall of said conduit means to said organic working fluid.
 2. A vaporgenerator according to claim 1 wherein said annular enclosure comprisesconcentric side walls and top and bottom annular end walls, furthercomprising: a. an annular exhaust passage substantially coextensive withsaid annular enclosure and extending downwardly from said bottom endwall; b. means forming at least one entrance opening between saidcombustion chamber and said exhaust passage to admit combustion productsto said exhaust passage; and c. means in said exhaust passage forming atleast one exit opening remote from said entrance opening for dischargingexhaust gases, whereby exhaust gases travel from said combustionchamber, along a path adjacent the bottom end wall of said enclosure,before being discharged.
 3. A vapor generatr according to claim 2wherein said exit opening is situated 180* from said entrance opening.4. A vapor generator according to claim 2 further comprising: a. heattransfer fins extending upward from said bottom end wall into said heattransfer fluid; and b. heat transfer fins extending doWnward from saidbottom end wall into said exhaust passage.
 5. A vapor generatoraccording to claim 4 wherein said downwardly extending heat transferfins comprise elongated plate-like members for directing the flow pathin said exhaust passage.
 6. A vapor generator according to claim 2further comprising thermal insulation along the common wall between saidenclosure and said annular space restricted substantially to the portionof said common wall above the level of liquid in said enclosure.
 7. Avapor generator according to claim 1 further comprising means responsiveto the pressure in said enclosure for controlling the heat input to saidheat transfer fluid, to thereby maintain a predetermined operatingtemperature level in said vapor generator.
 8. A vapor generatoraccording to claim 1 further comprising: a. pressure actuated means forcontrolling the heat input to said heat transfer fluid; b. means forapplying a signal proportional to the pressure internal of saidenclosure to said pressure actuated means, said signal providing thetotal information required for enabling said pressure actuated means tovariably control said heat input and maintain working fluid vapordischarged from said vapor generator at a predetermined temperaturelevel.
 9. A vapor generator for closed cycle vapor engine utilizing athermally sensitive organic working fluid, said vapor generatorcomprising: a. means forming a substantially rigid, sealed, fluid-tightenclosure of substantially constant volume, substantially devoid offluids which are non-condensable over the working temperature range ofsaid vapor generator, partially filled throughout said range with a heattransfer fluid in the liquid state, said heat transfer fluid beingnon-decomposable throughout said range; b. heat source means forvaporizing a portion of said heat transfer fluid; c. conduit meansextending through said enclosure with at least a portion thereof out ofcontact with said heat transfer fluid in the liquid state and forming acondensing surface upon which vaporized heat transfer fluid condenses totransfer thereto its latent heat of vaporization, said conduit meanshaving an inlet for admitting working fluid into heat exchangerelationship with the interior of said enclosure and an outlet fordischarging working fluid from said heat exchange relationship; and d.pressure responsive control means, responsive only to the vapor pressurewithin said enclosure, coupled to said heat source means for controllingthe output thereof to maintain the heat transfer fluid vapor, andthereby the entire condensing surface, at a temperature level not lowerthan the boiling point of said working fluid and below the temperatureat which an unacceptable amount of thermal degradation of said workingfluid occurs, whereby said vapor generator is enabled to heat workingfluid at said outlet to vaporization without producing thermaldecomposition thereof.
 10. A heat transfer device according to claim 9wherein said heat transfer fluid comprises biphenyl and biphenyl oxide.11. A heat transfer device according to claim 9 wherein said pressureresponsive means comprises a variable control for selecting theoperating temperature of said condensing surface.